Gene flow and recombination directly oppose genetic differentiation and speciation because they homogenize allele combinations that are unique to each population/species (Felsenstein 1981; Rieseberg 2001; Ortiz-Barrientos, Reiland et al. 2002; Butlin 2005). Under divergence with gene flow, either very strong selection or some other mechanism that reduces recombination rates is needed to maintain different allelic combinations.
Decreased recombination is especially important in cases of ecological speciation with gene flow. A decade of work in evolutionary biology indicates that divergent natural selection in different habitats is usually involved in speciation (i.e. ecological speciation). While ecological selection alone can promote speciation (Schluter 2001; Rundle and Nosil 2005; Schluter and Conte 2009), a variety of species contain populations that are adapted to different habitats and yet have not undergone speciation (Hendry 2009; Nosil, Harmon et al. 2009). These conflicting outcomes suggest that ecological selection need not always lead to the evolution of reproductive isolating barriers (RIBs). When the genes under ecological selection also confer reproductive isolation, ecological speciation occurs readily (Boughman 2001; Kirkpatrick and Ravigne 2002; Gavrilets 2004). However, when the genes under ecological selection are independent of the genes conferring reproductive isolation, alleles for RIBs must somehow become associated with the alleles under ecological selection, thus providing a link between adaptation and reproductive isolation (Felsenstein 1981; Servedio 2009). To date, few studies have examined the mechanisms by which divergent natural selection can lead to the development of reproductive isolation.
I examined role of divergent natural selection in speciation using the sister species Lucania goodei and L. parva (Duggins, Karlin et al. 1983; Whitehead 2010). Lucania goodei and L. parva are differentially adapted to salinity with L. goodei being primarily freshwater and L. parva being euryhaline (able to tolerate a wide range of salinities). In my thesis I determined the extent to which divergent natural selection (with regard to salinity tolerance) has led to reproductive isolation.
First, I searched for physiological mechanisms to explain the difference in salinity tolerance between the two species. I did this by exposing individuals of both species to different salinities and then performing a real time PCR study to examine changes in mRNA transcript levels of genes known to be involved in osmoregulation. I found that L. parva expressed higher levels of the genes involved in saltwater ion/osmoregulatory regulation than did L. goodei, but that both species expressed similar levels for two of the three genes involved in freshwater osmoregulation.
Next, I examined whether or not divergent natural selection for salinity tolerance has led directly to behavioral isolation (Chapter 3). Previous work indicated that behavioral isolation was present, but did not test whether the expression of behavioral isolation varied with environmental conditions. Adaptation to different salinities may have led to differences in mating signals and perception of those signals, producing behavioral isolation as a byproduct. I tested if this had occurred by manipulating the environment the fish were in. If mating signals and preferences in L. parva are especially adapted to saltwater and those of L. goodei to freshwater, then changing the salinity should decrease the strength of behavioral isolation by interfering with these signals.
The results of this study suggested that divergent natural selection did not play a role in the evolution of behavioral isolation. However, there was a striking asymmetry in courtship between the species that was concordant with the direction of intrinsic isolation. Hence, there may have been selection for species recognition in male L. goodei due to low fitness in hybrids (i.e. reinforcement).
The classic signature of reinforcement is heightened behavioral isolation in areas of sympatry. In Chapter 4, I explicitly tested whether or not reinforcement of male preferences had occurred by comparing male preferences from sympatric and allopatric populations. Males from sympatric and allopatric populations were exposed to females of both species over two days and their behavior was monitored. Allopatric males were significantly more likely to court heterospecific females than sympatric males. This is in agreement with the expectations of reinforcement theory.
In the final two chapters of my thesis, I tested whether or not chromosomal rearrangements have led to linkage disequilibrium between genes underlying reproductive isolation and genes underlying salinity tolerance. Many closely related species differ in chromosome number and shape. Chromosomal rearrangements can be important to both adaptation and the development of reproductive isolation since rearrangements can physically link species-specific genes in areas of low recombination (White 1978; Trickett and Butlin 1994; Noor, Grams et al. 2001; Rieseberg 2001; Ortiz-Barrientos, Reiland et al. 2002; Feder, Roethele et al. 2003; Navarro and Barton 2003; Brown, Burk et al. 2004; Butlin 2005; Kirkpatrick and Barton 2006; Carneiro, Ferrand et al. 2009). I created linkage maps for both L. goodei and L. parva and analyzed them for synteny to determine if genomic rearrangements had occurred (Chapter 5). A fusion between linkage groups 13 and 14 in L. goodei has led to a large metacentric chromosome (linkage group 1 in L. parva). I performed a QTL analysis to determine if the genes underlying salinity tolerance and reproductive isolation are located within this rearrangement (Chapter 6). The fusion contained QTLs for multiple traits involved in reproductive isolation. In contrast, the genetic underpinnings of salinity tolerance had little relationship to reproductive isolation. Only one of four QTL underlying salinity tolerance mapped to the same location as a QTL involved in reproductive isolation. Thus, genetic differentiation in salinity tolerance does not seem to be strongly associated with reproductive isolation. Instead, my data suggest that chromosomal rearrangements are important in linking female and male behavioral isolation and genes related in intrinsic isolation. Linkage of behavioral isolation with intrinsic isolation is vital for reinforcement (Ortiz-Barrientos, Grealy et al. 2009). The chromosomal fusion may have aided reinforcement in this system by bringing loci underlying behavioral isolation and intrinsic isolation into linkage disequilibrium.
My dissertation indicated that divergent natural selection likely had little to do with speciation in this group and future work should concentrate on discerning the non-ecological mechanisms (i.e. reinforcement) that led to reproductive isolation.